There are various types of displaying systems allowing a synthetic image to be superposed on an exterior environment. One possible solution illustrated in FIG. 1 consists in implementing a stereoscopic image projector. The displaying system 10 then comprises:                what is called a “3D” stereoscopic image projector 11 capable of generating at least two images, called the “right eye”/“left eye” image, representative of a given object. In the case of FIG. 1, the object is a sphere S.        a scattering semi-transparent screen 12 onto which the “right eye”/“left eye” images are projected;        a pair of glasses 13 including means for separating the “right eye”/“left eye” images and first detecting means 14, and intended to be worn by a user;        second detecting means 15 associated with a fixed coordinate system R and that, associated with the first detecting means 14, allow the spatial position of the pair of glasses 13 in this fixed coordinate system to be detected;        an electronic processor 16 performing at least the following functions:                    acquisition of signals generated by the detecting means 14 and/or 15 and computation of the position of the pair of glasses;            computation of the position of the stereoscopic image corresponding to the position of the pair of glasses;            computation of the two right eye/left eye images.                        
There are various means for ensuring the stereoscopic separation of the projected images.
In a first technical solution, temporal separation is used. The projector sequentially projects and in a synchronized manner first the right eye image then the left eye image. The glasses are active and include active shutters that are synchronized with the projector. Thus, each eye perceives the image that is intended therefor and only said image. The shutters are generally based on a liquid-crystal technology. This solution has a number of drawbacks.
Since the glasses are active they require a power supply and control electronics, this posing maintenance problems in the context of an on-board use. In addition, the polarizers of the LCD shutters cause darkening of the cockpit displays and may even completely occult them, depending on the various polarization directions and the inclination of the glasses. Lastly, the presence of polarizers and the left-right vision alternation required to achieve the stereoscopic vision leads to a substantial loss of light. Thus the transmission of the glasses does not exceed 30%, this causing an unacceptable darkening of the exterior landscape.
In a second technical solution, the stereoscopic projector operates in a polarized mode. It emits successively and periodically a right eye image with a first polarization and a left eye image with a second polarization that is different from the first polarization. The pair of glasses 13 is passive. It includes a first polarized lens that is transparent to the first polarization and opaque to the second polarization, and a second polarized lens that is transparent to the second polarization and opaque to the first.
Such polarizer glasses are passive and solve the problem of alternate occultation of each eye, and the management of batteries. In contrast, it is absolutely necessary to use a silvered projection screen that preserves polarization. Since such screens are not transparent, they are not suitable for the applications to which the invention relates.
In a third technical solution, the projector emits two colour images the emission spectra of which are distinct. The pair of glasses includes two different filters, the first transmits the first spectrum and filters the second spectrum. The second filter has the inverse function. Thus, each eye perceives one and only one colour image and only said image. This technique is known as anaglyph. The simplest way of producing an anaglyph is to separate the visible spectrum into two portions, one red and the other blue. The obvious advantage of the device is its great implementational simplicity, but vision of the exterior world is greatly degraded.
More advanced, the system referred to as spectral multiplexing separates the visible spectrum into two interlaced portions, one dedicated to each eye. However, if the colours of the landscape are to be preserved as well can be, luminance is considerably decreased. The patent applications of the company “Dolby Laboratories Licensing Corporation” US 2011/0205494, US 2013/0342904 and US 2014/0022637 describe solutions of this type for cinematographical applications that require neither high light levels nor, of course, an exterior landscape to be transmitted.
For a certain number of applications, the use of colour images is not necessary. In the field of the superposition of images on an exterior landscape, it may be preferable to use a monochromic symbology that will stand out perfectly on the exterior background rather than a colour image that risks introducing confusion into the perception of the landscape. The image projector according to the invention implements monochromic stereoscopic images emitted at wavelengths that are different but sufficiently dose to give the same visual colour sensation. One of the difficulties of this method consists in finding light sources that are similar enough that their visual appearance is substantially the same and that are sufficiently separate that they are spectrally separable without difficulty and without excessive efficiency loss. Lastly, these light sources must meet aeronautical standards.
A first solution, illustrated in FIG. 2, is based on the spectral transmission properties of interference filters as a function of incidence. The projector 11 then includes:                an interference filter 22 placed in front of the imager 20, which is illuminated by a source 21 the spectral transmission of which includes at least one transmission band of set width centred on a wavelength, said wavelength depending on the incidence of the light on said interference filter, and;        means, symbolized by the semi-circular arrow, allowing the angular position of the filter to be varied between two set positions so as to transmit, in the first position, a first spectral band and, in the second position, a second spectral band.        
A second solution is illustrated in FIG. 3. It consists in using two light sources 31 and 35 that are identical but filtered differently so as to let two neighbouring spectral bands pass. The first source 31, which is filtered by the filter 32, illuminates a first imager 30 and first optics 33, this first assembly being dedicated to the formation of the right stereoscopic image, and the second source 35, which is filtered by the filter 36, illuminates a second imager 34 and second optics 37, this second assembly being dedicated to the formation of the left stereoscopic image.